Microsc. Microanal. 23, 329–335, 2017 doi:10.1017/S1431927617000149
© MICROSCOPY SOCIETY OF AMERICA 2017
Understanding of Capping Effects on the Tip Shape Evolution and on the Atom Probe Data of Bulk LaAlO3 Using Transmission Electron Microscopy
Chang-Min Kwak,1 Young-Tae Kim,1 Chan-Gyung Park,1,2 and Jae-Bok Seol2,*
1Department of Materials Science and Engineering, POSTECH, Pohang 790-784, South Korea 2National Institute for Nanomaterials Technology (NINT), POSTECH, Pohang 790-784, South Korea
Abstract: Two challenges exist in laser-assisted atom probe tomography (APT). First, a drastic decline in mass-resolving power is caused, not only by laser-induced thermal effects on the APT tips of bulk oxide materials, but also the associated asymmetric evaporation behavior; second, the field evaporation mechanisms of bulk oxide tips under laser illumination are still unclear due to the complex relations between laser pulse and oxide materials. In this study, both phenomena were investigated by depositing Ni- and Co-capping layers onto the bulk LaAlO3 tips, and using stepwise APT analysis with transmission electron microscopy (TEM) observation of the tip shapes. By employing the metallic capping, the heating at the surface of the oxide tips during APT analysis became more symmetrical, thereby enabling a high mass-resolving power in the mass spectrum. In addition, the stepwise microscopy technique visualized tip shape evolution during APT analysis, thereby accounting for evaporation sequences at the tip surface. The combination of “capping” and “stepwise APT with TEM,” is applicable to any nonconductors; it provides a direct observation of tip shape evolution, allows determination of the field evaporation strength of oxides, and facilitates understanding of the effects of ultrafast laser illumination on an oxide tip.
Key words: LaAlO3, metallic capping, atom probe tomography, transmission electron microscopy, correlative analysis
INTRODUCTION
Atom probe tomography (APT), which has been sig- nificantly developed over the past decade, allows accurate three-dimensional (3D) positioning of atoms with near- atomic resolution and identification of the chemical nature of each atom in a material system (Geiser et al., 2007; Kelly & Larson, 2012). APT was used to analyze the chemistry of pure metals or metallic engineering alloys in which solute or trace elements segregate to material’s inter- nal structural defects (Blavette et al., 1999; Miller, 2006), interfaces (Guo et al., 2014; Seol & Kim, 2016), or pre- cipitates (Knipling et al., 2007; Seol et al., 2013; Yao et al., 2016). However, the development of laser-assisted APT has widened the field of application to include nonconductive
materials and heterogeneous structures including NiO thin films (Yoon et al., 2008) and SiO2 insulators (Kwak et al., 2016). Moreover, some outstanding observations have been reported, including the discovery that the bandgap in an insulator approaches zero in a high field, so the insulator becomes essentially metallized (Silaeva et al., 2014). The physical mechanisms that explain the field evaporation in nonconductive materials with laser-pulsed APT include photoionization and thermally induced evaporation
*Corresponding author.
jb_seol@postech.ac.kr Received June 29, 2016; accepted January 15, 2017
(Vurpillot et al., 2007; Marquis et al., 2010; Vella et al., 2011; Kelly et al., 2014; Silaeva et al., 2014). However, the apex of the tip becomes severely asym-
metric during APT analysis as a result of inhomogeneous heating of the side illuminated by the laser (Houard et al., 2011; Koelling et al., 2011). This phenomenon leads to an asymmetric field evaporation or a difference in the tip shape between the laser-irradiated side and the opposite side (shadow side) when using relatively high laser energies. The ions emitted from the shadow side need a few nanoseconds to become thermally activated, due to the low thermal dif- fusivity of oxide materials. As a result, the field evaporation of the shadow side is delayed, leading to a lowering of the mass-resolving power in the mass spectrum. To avoid this laser-induced asymmetric evolution of
APT tips, the needle-like tips can be metal capped to increase the uniformity of heat flow in the entire surface regions (Larson et al., 2013). More recently, our previous results demonstrated that the use of Ni-, Co-, and Ag-capping layers on MgO oxide tips improved the mass-resolving power and the signal-to-noise ratio, and also reduced the number of multiple hit events (Seol et al., 2016). Specifically, the Ag-capping layer profoundly improved the reliability of APT data from bulk MgO, and we suggested that the metal capping on bulk oxide tips leads to fast heat diffusion. However, experimental evidence to explain the function of such capping effects on other oxide materials is lacking, and
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